data fetched twice at initialization using dynamic size

[Zecat]

To reproduce:

• Let the default size to 0
• Inform size dynamically using _dataSource callback second parameter

What happens:
_dataSource is called twice = request duplication

Filling issue:
-> _dataSource callback is called
-> size updated
-> _sizeChanged is triggered
-> (size > oldSize) is true because oldSize = 0
-> refreshPage is called

Problem:
data should not be refreshed when oldSize = 0

I propose to change https://github.com/Saulis/iron-data-table/blob/master/iron-data-table.html#L881

if (size > oldSize) {

with

if (oldSize && size > oldSize) {

nice element!